(CNN) -- Reports of young children developing premature or inappropriate sex characteristics have prompted the U.S. Food and Drug Administration to require a boxed label warning on two testosterone gel products.

Men use testosterone gels if they no longer produce testosterone or produce it in low amounts.

The FDA announced Thursday that these children had experienced adverse effects in growth and sexual development after coming into contact with adults who used the products.

The gels, AndroGel and Testim, are approved for use in men who produce little or no testosterone. In 2007, there were 1.4 million prescriptions dispensed for AndroGel -- 25,000 of which were for women -- and 370,000 for Testim.

Some women use testosterone gel products off-label, perhaps to increase sex drive, but the FDA has not approved these products for any reason in women.

Other non-FDA approved products are sold over the Internet. The agency has not reviewed the safety and efficacy of these products anddoes not know how widely they are sold.

In explaining how a child might have been exposed, Dr. Dianne Murphy, director of the Office of Pediatric Therapeutics at the FDA, suggested that a gel user might forget to wash his or her hands or cover treated areas, or may have not waited for skin to dry, and then picked up the child and held him or her to the upper body.

The ratio of skin surface area to total body size of a child is much greater than in an adult, Murphy said. That means that a small area on an adult's body corresponds to a greater percentage of surface area on a child's body.

The FDA has reviewed eight cases of children ages 9 months to 5 years who developed symptoms as a result of exposure to the gel up to December 1. Abnormal effects included inappropriate enlargement of the genitalia, premature development of pubic hair, advanced bone age, increased libido and aggressive behavior.

Although most children's signs and symptoms regressed after they were no longer exposed to the gel, in a few cases the enlarged genitalia did not return to normal, and the bone age was still higher than the child's chronological age, the FDA said.

One of these children required surgical intervention, the FDA said. Some children also had to undergo invasive diagnostic procedures.

More than a dozen additional reported cases are currently under review by FDA.

Androgel is applied to the upper arms and shoulders and abdomen once per day, and Testim is applied to the upper arms and shoulders once per day, said Dr. Mark Hirsch of the FDA. The current labels instruct users to wash their hands after use, and to cover treated skin with clothing.